Collectorless d. c. motor

ABSTRACT

A collectorless D.C. motor includes a rotor, a stator, and a plurality of stator windings arranged on the stator in angularly spaced relationship. Each winding when energized produces a stationary magnetic field having a respective orientation. A control circuit including an oscillator energizes successive ones of the windings for respective time intervals determined by the oscillatory frequency of the oscillator. The resulting magnetic field revolves with an angular velocity determined by the oscillator.

United States Patent Schaub 14 1 Mar. 5, 1974 [54] COLLECTORLESS D. C.MOTOR 3,662,237 5/1972 Favre 318/138 3,683,248 8/1972 K0bayashi..318/138 [751 lnvemor- Gerhard schaub Nuremberg 3,368,128 2/1968 Parrish318/326 Germany 3,290,572 12/1966 Hartmanm. 318/326 3,584,280 6/1971lnagaki 318/138 1 Asslgneegz g g g gi r fg' GmbH 3,577,057 5/1971 Dyer318/341 Filed: 1972 Primary Examiner-Hemard A. Gilheany AssistantExaminerThomas Lan er 21 A LN 236707 8 1 pp 0 Attorney, Agent, orFirm-Michael S. Striker [30] Foreign Application Priority Data 57]ABSTRACT pr 1 1 Germany A collectorless DC. motor includes a rotor, astator, 52 US. Cl. 318/138 and a plurality 0f Stat" windings arrangedthe 51 Int. Cl. H02k 29/02 angulafly spaced relafimship- Each winding 5Field of 318,138 2 7 341 331 when energized produces a stationarymagnetic field 318/254: having a respective orientation. A controlcircuit including an oscillator energizes successive ones of the [56]References Cited windings for respective time intervals determined bythe oscillatory frequency of the 0sci1lator. The result- UNITED STATESPATENTS ing magnetic field revolves with an angular velocity 5:12:18determined by the oscillaton 31611981 10/1971 Watson 318 138 15 Claims,2 Drawing Figures PATENTEBHAR 5 I974 SHEET 1 BF 2 D. C. vo l-l-a e sourr N m .m e w s r k l m n '6 4g 4 e 2 v m 4 B 1 PATENTEB MAR 51974 SHEET2 1F 2 w 1 M i Q MTQNTQ MNLWTUS MNLNK Ill E E m E; m

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COLLECTORLESS D. C. MOTOR BACKGROUND OF THE INVENTION- The inventionrelates to DC. motors, and particularly to collectorless DC. motors ofthe revolving-field type.

Still more particularly, the invention relates to collectorless D.C.motors which comprise a permanent magnet rotor and a multi-pole statorprovided with a plurality of stator windings, each of which is connectedto and disconnected from a D.C. source by means of an electric controlcircuit.

Collectorless DC. motors are already known. In general, they comprisemeans for establishing a revolving magnetic field and a permanentlymagnetic, or otherwise magnetized, rotor which follows such revolvingfield. These constructions have employed a plurality of discrete statorwindings providedaround the periphery of the stator of the motor anddistributed at equal angular intervals. The revolving magnetic field wasproduced by connecting successive ones of the stator windings to a D.C.voltage source, the windings being connected in succession so as toproduce an overall magnetic field whose net orientation rotates in thedesired direction. The energization of successive windings wasaccomplished by means of electronic control circuitry, and sometimesalso by mechanical means.

Several constructions are known. In one such construction, an auxiliarycollector controls the energization of successive stator windings as afunction of rotor position. This approach is analogous to conventionalmechanical commutation in ordinary DC. motors, inasmuch as the controlof stator winding current is effected as a direct function of rotorposition.

Auxiliary collectors according to the prior art have been available inseveral forms. According to prior-art constructions, the auxiliarycollector comprised Hall generators, field plates, or other inductivepick-ups distributed about the periphery of the rotor in angularcorrespondence to the distribution of the several stator windings. Theauxiliary collector has also been provided in form of a mechanicalsliding-contact or wiper. Such mechanical wipers guarantee a highstarting torque and conventionally, when the desired rotor speed hasbeen reached, are lifted free of their tracks by a centrifugal actuatoror the like. It will be appreciated, of course, that such mechanicalsliding-contacts, because of their susceptibility to wear, are hardlymore advantageous than conventional brushes of commutated D.C. motors.

SUMMARY OF THE INVENTION It is one object of the present invention toovercome the disadvantages of prior-art uncommutated D.C. motors,particularly those requiring mechanical auxiliary collectors.

It is another object of the invention to provide a DC. motor in which aplurality of stator windings are provided on the stator and aresuccessively energized, whereby to establish a revolving magnetic field.

It is still a further object to provide such a motor wherein the statorwindings are successively energized and tie-energized independently ofrotor position.

It is yet another object of the invention to establish a revolvingmagnetic field in a DC. motor without the use of mechanical collectors,auxiliary collectors, or

commutators, and solely by electrical and electronic means.

It is still a further object of the invention to provide a collectorlessDC. motor having as nearly as possible the characteristics of ashunt-wound DC motor.

It is yet another object to provide such a motor wherein the statorwindings are successively energized for respective time intervals whichare predetermined and adjustable.

It is an additional object to provide such a motor as a speed-variablesynchronous motor.

It is a further object to provide such a motor as a speed-variablesynchronous motor whose synchronous speed automatically rises from zeroto a desired value without operator control of the actual start-upoperation.

his a further object to provide such a motor with automaticspeed-regulation capability.

These objects, and others which will become apparent from the followingdescription, are met by a construction for a collectorless DC. motorwhich comprises a rotor, a stator, a plurality of stator windingsarranged on the stator in angularly spaced relationship and eachproducing when energized a stationary magnetic field having a respectiveorientation. Furthermore, control circuit means including an oscillatorcircuit controls energization of successive ones of the stator windingsfor respective time intervals determined by the oscillatory frequency ofthe oscillator. In this way, a revolving magnetic field is producedwhose angular velocity is determined by the oscillatory frequency of theoscillator.

The motor according to the invention is characterized by extremeflexibility and completeness of control, it having if desired theoperating characteristics of a shunt motor, and its various operatingparameters being adjustable according to the needs of specificapplications.

In the steady-state the motor according to the invention operates as asynchronous motor, because the frequency of the revolving magnetic fieldis electrically determined. On the other hand, the motor of theinventionhas the characteristic of a shunt-wound motor, in that it isspeed-adjustable while having a speed that is largely independent of thetorque which it must develop during use.

In particular, the control of the field frequency by means of anoscillator is most advantageous, because it permits simple start-up. Thestart-up frequency of the motor can be made very low, by suitablyadjusting the oscillator circuit, and then gradually increased, wherebyto permit gradual acceleration of the rotor. In this way, although mymotor operates in the steadystate as a synchronous motor, it does notrequire a separate start-up winding or an inconveneint start-upprocedure, the motor being capable of synchronous or nearly synchronousoperation over the complete range of speeds to be employed.

One particularly advantageous embodiment according to my inventionincorporates a ringcounter circuit having an input connected to theoscillator circuit and a plurality of outputs, each connected with arespective power transistor. Each power transistor is connected with andcontrols the flow of current through a respective stator winding and inthis way the ring counter, through the intermediary of the respectivepower transistors, accomplishes the energization of successive ones ofthe stator windings, thereby establishing a revolving stator field.Utilizing this expedient, the time interval for which respectivewindings are energized, and the rate of transitions of state from onering counter output to the next, is determined by the oscillatoryfrequency of the oscillator, whose pulses serve to drive the ringcounter. Advantageously, though merely by way of example, the oscillatormay be provided in form of an adjustable multivibrator.

I also contemplate the provision of a novel start-up arrangement, bymeans of which the motor is startedup automatically, without operatorcontrol of the acceleration. According to this concept, a feedbacknetwork feeds back to the oscillator a voltage corresponding to therotational speed of the rotor. The magnitude, sign, and/or frequency ofsuch speed-signal voltage serves to vary the oscillatory frequency ofthe oscillator. As one possibility, I contemplate feeding back to theoscillator a voltage corresponding to rotor speed, the sign of suchvoltage producing a corresponding increase or decrease in theoscillatory frequency, this increase or decrease advantageously beingproportional to the magnitude of the fed-back voltage. In this way, alow start-up frequency for the oscillator may be set, to establish aslowly revolving field and to permit startup of the rotor. As the rotorspeed increases, and by provision of such regenerative feedback network,the oscillator frequently will also increase, thereby causing furtheracceleration of the rotor. Such process continues automatically,resulting in higher and higher speeds. When utilizing such construction,I provide speed-limiting means, which advantageously may be adjustable,and which may serve to limit the actual speed of the rotoror thefrequency of the oscillator or the magnitude of the fed-backregenerative voltage. With the provision of such means, when the desiredsynchronous speed has been attained by the rotor, acceleration isdiscontinued, and steadystate operation I commences. Such start-upacceleration of the rotor may easily be programmed by suitable choice ofthe feedback network or in other manner, and can be performed completelyautomatically, without operator control. I have found it a simplematter, utilizing this method of feedback, to establish operatingcharacteristics for the motor which closely resemble those ofconventional shunt-wound D.C. motors.

In addition to such automatic variation of the oscillator frequencyduring start-up, I also contemplate the variation of oscillatorfrequency at the will of an operator. Specifically, l contemplateadjustable setting of the initial start-up frequency of the oscillatorfor purposes of rotor start-up, as well as adjustable setting of thesteady-state oscillator frequency corresponding to the steady-staterotational speed of the rotor. Advantageously, and quite simply, I canrealize such control by connecting the oscillator to the D.C. voltagesource for the motor by means of an adjustable RC-network, thisRC-network serving as frequency-determining element. When the oscillatoris constructed as a conventional multivibrator, for instance, thefrequencyvarying means may comprise adjustable resistor and/or capacitormeans forming part of the multivibrator, and may also comprisevoltage-varying means for varying the trigger voltages associated withthe multivibrator.

For rotor-position-dependent control of winding energization after startup of the D.C. motor, the oscillator can be connected with the statorwindings by means of an auxiliary feedback conductor and associatedZener diodes, or by means of other feedback networks. The Zener diodescan be so arranged, and their breakdown voltages so coordinated with themagnitude of voltages induced by the moving rotor in the stator windingsof the stator, that when the preselected upper rotor speed has beenreached the Zener diodes will be rendered conductive and willeffectively disconnect the oscillator from the remainder of the controlcircuit, e.g., from the ring counter, if such is used. It is furthermore possible to connect each armature winding, preferably via diodes,with a respective stage of the ring counter, or other control circuitmeans, in in-phase relationship. By means of such a circuit arrangementthe operating characteristics of the motor will particularly closelyresemble those of a conventional shunt-wound D.C. motor.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof,

' will be best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates in schematic formsome of the principles which underlie the invention; and

FIG. 2 illustrates one exemplary control circuit according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I FIG. 1 is a generalized blockdiagram ofa motor and control circuit according to the invention. Themotor itself is illustrated schematically and identified by referencecharacter M, and it comprises a permanently magnetized rotor 11a and astator 10. Stator 10 is provided with three angularly spaced statorwindings, illustrated schematically in the Figure and respectivelyidentified by reference characters a, b, 0. Each of windings a, b, 0produces when energized a stationary magnetic field having a respectiveorientation.

The control circuit means according to this representation compriseswinding selector l2, oscillator circuit 13, speed limiter 14, adjustableRC-network 18, as well as their interconnections. It will be seen thatthe several armature windings are respectively connectable to D.C.voltage source 11 by means of winding selector l2. Winding selector 12is connected to oscillator 13.by means of conductor 16. During operationof the motor, winding selector 12 energizes successive ones of theangularly spaced windings a, b, c for respective time intervalsdetermined by the oscillatory frequency of oscillator circuit 13. Inthis way, the control circuit, stator windings and D.C. voltage sourcecooperate to establish a net stator field which revolves with an angularfrequency determined by the frequency of oscillator circuit 13.

- Feedback conductor l5--which need not be a simple conductor, but canbe whatever feedback network is desired for a particularapplication-feeds back from motor M a speed signal voltage correspondingto the angular speed of rotor 119. This fed-back voltage may correspondto rotor speed in its frequency and/or its magnitude, or in anothermanner, and is fed back to speed limiter/controller 14, associated withoscillator 13.

Speed limiter/controller 14 serves to limit the angular speed of rotor18 to a predetermined maximum. According to the invention, limiter 14may accomplish this purpose in any of several ways. It may incorporate athreshold detector which, when the frequency and/or magnitude of thespeed signal has reached a predetermined maximum, serves to disconnectoscillator 13. from supply 11, or from winding selector 12, or which mayterminate operation of oscillator 13 in some other manner.

Alternatively, I contemplate speed limiter means which, instead ofdirectly monitoring the actual rotor speed, may serve to limit thefrequency of the oscillator, and in that way regulate the speed of therotor.

As a further alternative, speed limiter/controller 14 may serve tocontinuously control the oscillator frequency as a function of rotorspeed, and not merely limit the rotor speed and/or oscillator frequencyto a predetermined upper limit. This possibility is more clearly setforth in FIG. 2.

Adjustable RC-network 18 is illustrated in FIG. 1 to represent thepossibility of adjustably controlling the oscillatory frequency ofoscillator circuit 13, over and above control of such frequency viaautomatic speed controller/limiter 14.

symbolically depicted feedback network 17 represents the possibility offeeding back to winding selector 12 synchronization signalscorresponding to rotor speed and/or rotor position, thereby bypassingoscillator 13.

A concrete control circuit according to the invention, incorporatingmany of the features outlined above, is illustrated in FIG. 2. Veryadvantageously in this Figure, but merely by way of example, windingselector 12 is provided in form of a conventional ring counter having,in this embodiment, three circuit stages. Each stage includes a bistablemultivibrator comprising two transistors 121, 122, and each stage hasassociated with it a respective one of power transistors 20, 21 and 22.The emitter of each such power transistor is connected to negativesupply terminal 111. The collector of each transistor 20, 21, 22 isconnected to one end ofa respective stator winding 101, 102 or 103, theother ends of which are connected to positive supply terminal 112. Eachbistable multivibrator controls the conduction state of the respectiveone of power transistors 20, 21, 22, which latter in turn serve toconnect or disconnect the respective stator winding with the supply.

The triggering pulses for ring counter 12 are furnished by triggering 13through the intermediary of coupling stage 19. In this embodiment,triggering means 13 comprises a conventional astable multivibratorincluding two transistors 131, 132 and various resistances andcapacitances, whose respective values determine, in the usual manner,the on and off times of each transistor, and thus the overall signalreptition frequency.

It will be seen that, in this embodiment, astable multivibrator 13comprises two adjustable capacitors forming part of frequency-varyingmeans and permitting adjustment of the respective onand off-times oftransistors 131 and 132, and thereby permitting adjustment of theoverall oscillatory frequency. Clearly, the provision of adjustablecapacitors is merely exemplary, it being possible and usually preferableto make one of the multivibrator resistances adjustable.

The operation of that part of the circuit thus far described will beself-evident. When supply terminals 111, 112 are connected to power,oscillator 13 will commence to oscillate at a predetermined frequency.The pulses generated by oscillator 13 will be transmitted to and drivering counter 12, through the intermediary of coupling stage 19.Successive ones of the three ring counter stages will be brought to acondition permitting conduction of the respective power transistor,thereby resulting in energization of successive ones of stator windings101, 102, 103. In this way, a revolving magnetic field will be produced,which the rotor of the motor will follow synchronously.

According to the embodiment of FIG. 2, provision is additionally madefor an automatically controlled startup operation. For this purpose, thefrequency-varying means of FIG. 2 further includes three diodes 23,which form part of speed-signal means, each being connected to afeedback conductor 15, which in turn is connected to control point 133and applies a feedback voltage to the circuitry of the astablemultivibrator. As will be appreciated by those skilled in the art,during running of the motor a voltage is induced in each of statorwindings a, b, and c, this induced voltage being proportional to thespeed of the rotor and having a sense which in the illustrated circuitalters the overall voltage across the respective stator windingterminals. Frequencyvarying diodes 23 are each so connected to oneterminal of a respective stator winding 101, 102 or 103 that they each,via feedback conductor 15, feed back to control point 133 a voltagewhose magnitude changes in proportion to changes of rotor speed. Thisspeed signal voltage is smoothed by the capacitor connected betweenterminals 112, 133 and is applied to the internal circuitry ofmultivibrator 13. Depending on the magni-- tude of the feedback speedsignal voltage, the respective one of transistors 131, 132 is triggeredsooner or later, its on or off time being accordingly altered, therebyresulting in a change in the overall oscillatory frequency of theoscillator.

The operation with this part of the circuit taken into account is asfollows. When supply terminals 111, 112 are connected to a source of DC.voltage, oscillator 13 commences to oscillate at a predeterminedstart-up frequency, which may be adjustably pre-set by means of theillustrated adjustable capacitors. This start-up frequency willordinarily be quite low, so as to permit the acceleration fromstandstill of the rotor. As the rotor speed increases, the voltageinduced in the respective stator windings 101, 102, 103 is communicatedto control point 133 via diodes 23 and feedback conductor 15. The changein net voltage across windings 101, 102, 103 will make itself felt atcontrol point 133 as a voltage change. This voltage increase results inearlier transition of the multivibrator from the one to the other of itsstates, and thereby results in an overall increase of the oscillatoryfrequency. The increased oscillatory frequency causes the revolvingstator field to revolve at a somewhat higher speed, resulting in furtheracceleration of the rotor. This regenerative feedback accordinglyassures that the rotor will be brought from standstill to a substantialspeed in an automatic operation, the increases of speed of the revolvingfield never exceeding the capacity of the rotor to acceleratecorrespondingly. Obviously, with this circuit arrangement the start-upprocedure becomes quite simple, not depending on the subjectivejudgement of the operator, and not requiring auxiliary start-up windingsor the like.

From what has just been said, it will be appreciated that the use ofregenerative feedback necessitates the provision of some sort ofspeed-limiting means. In the embodiment of FIG. 2 I providespeed-regulating means in the form of shut-off means which effectivelydisconnect the oscillator 13 from ring counter 12 when the rotor speedreaches a predetermined maximum value. Such shut-off means herecomprises three diodes 25 and three Zener diodes 24. As has already beenexplained, an increase of rotor speed makes itself felt as a voltagechange at control point 133, and thus also as a voltage change at theanodes of diodes 23 and 25. It will be seen that the circuits of Zenerdiodes 24 are connected between the lower-voltage terminal of windings101, 102, 103 and a circuit point 193. Accordingly, an increase of rotorspeed results in an increased voltage across Zener diode 24. When thisvoltage reaches the threshold value of the Zener diode, corresponding tothe desired maximum speed, the Zener diode becomes conductive. As aresult, and for as long as the rotor speed exceeds the predeterminedlimit value, the voltage between circuit points 133 and 1134 will befixed, effectively disconnecting the oscillator 13 from the ring counter12. In this way, speed regulation is achieved.

1 wish to emphasize that while, for the purposes of simplicity, l haveillustrated speed-regulating means including Zener diodes asthreshold-detecting components, other arrangements are of coursepossible, and

-in many cases preferable. In particular, for the sake of flexibility itmay be desired to provide thresholddetecting means which are adjustableand which respond to a threshold value selected at the will of theoperator.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions and circuit arrangements differing from the typesdescribed above.

While the invention has been illustrated and described as embodied in acollectorless DC. motor, it is not intended to be limited to the detailsshown, since various modifications and structural changes may be madewithout departing in any way from the spirit of the present invention.

In the embodiment of FIG. 2, a ring counter 112 is employed which has aplurality of stages, each connected with a single one of windings 101i,102, 103. However, such manner of connection is by no means the onlypossible one. Each stage of the ring counter may control the conductionof a plurality of power transistors, each such transistor connected to arespective stator winding. Alternatively, each power transistor couldserve for energization of a plurality of stator windingse.g-., for twoor more windings which are angularly spaced and in which when they areenergized a common current flows. Likewise, each stage of the ringcounter might control a plurality of transistors, each of which carriescurrent for a plurality of stator windings. There are of course manysimilar possibilities. In particular, it is emphasized that while inFIG. 2 a ring counter having three stages, corresponding to threetransistors and three stator windings, is used, a ring counter havingany number of stages may of course be employed. Furthermore, whereas inFIG. 2 only one stage of the ring counter at a time is in a state toenergize a winding, a ring counter or the like could equally well beemployed in'which at any given moment more than one stage is in a stateto energize a Winding, and in which several such states would travelsimultaneously about the circle of interconnected bistable elements, orthe equivalent.

Without further analyses, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. A collectorless DC. motor, comprising, in combination, a rotor; astator; a plurality of stator windings arranged on said stator inangularly spaced relationship and each producing when energized astationary magnetic field having a respective angular orientation;triggering means comprising free-running adjustablefrequency oscillatormeans having an output andoperative for furnishing at said output atrain of triggering signals having a signal repetition frequency, androtorspeed-responsive frequency-varying means connected to saidfree-running adjustable-frequency oscillator means and operative forcontinuously varying the frequency of said free-runningadjustable-frequency oscillator means in dependence upon the speed ofsaid rotor; and a ring counter ciruit having an input connected to saidoutput for receipt of triggering signals, and comprising a plurality ofcircuit stages each capable of undergoing transistions between a firstand a second state, and each stage being so connected to a respectiveone of said stator winding means as to effect energization of therespective one of said stator winding means when in said second state,and wherein said stages are connected together in a circle in such amanner that upon receipt of successive triggering pulses at said inputsuccessive ones of said stages undergo a transition to said secondstate, so that successive ones of said stator winding means areenergized in sequence to create a rotating magnetic field.

2. A motor as defined in claim ll, wherein said freerunningadjustable-frequency oscillator means consists of a free-runningadjustable-frequency oscillator having an input for receipt of a DC.signal and is operative for furnishing a train of triggering signalshaving a signal repetition frequency dependent upon the amplitude ofsuch DC. signal, and wherein said frequencyvarying means furthercomprises speed signal means for applying to said input of saidfree-running adjustable-frequency oscillator a DC. signal indicative ofrotor speed.

3. A motor as defined in claim 2, wherein said speed signal meansconsists of means for applying to said input of said free-runningadjustable-frequency oscillator a substantially flat DC. signal having alevel indicative of rotor speed.

4. A motor as defined in claim 1, said frequencyvarying means comprisingmeans for producing an increase in said signal repetition frequency inresponse to to an increase of rotor speed.

5. A motor as defined in claim 1, said frequencyvarying means comprisingmeans for establishing a signal repetition frequency having apredetermined startup value when said rotor is being started-up.

6. A motor as defined in claim 1, said triggering means comprisingadditional frequency-varying means for varying the signal repetitionfrequency of said train at the will of an operator.

7. A motor as defined in claim 6, said additional frequency-varyingmeans comprising an adjustable RC- network. i

8. A motor as defined in claim 1, said signal repetition frequencyhaving a start-up value when said rotor is being started-up, and saidtriggering means comprising means for adjustably determining saidstart-up value.

9. A motor as defined in claim 1, and further including speed regulatingmeans for regulating the speed of said rotor.

10. A motor as defined in claim 1, and further including shut-off meansfor terminating energization of successive ones of said winding meanswhen the rotor exceeds a predetermined value.

11. A motor as defined in claim 10, said shut-off means includingvoltage-threshold means connected with at least one of said stator forproducing a shut-off signal when the induced voltage in said at leastone stator winding reaches a threshold value.

12. A motor as defined in claim 11 said voltagethreshold meanscomprising Zener diode means.

13. A motor as defined in claim 1, said triggering means comprising amultivibrator.

14. A motor as defined in claim 1, said rotor comprising permanentmagnet means.

15. A motor as defined in claim 1, said circuit stages comprising aplurality of current-furnishin g power transistors, each connected witha respective one of said stator winding means.

1. A collectorless D.C. motor, comprising, in combination, a rotor; astator; a plurality of stator windings arranged on said stator inangularly spaced relationship and each producing when energized astationary magnetic field having a respective angular orientation;triggering means comprising free-running adjustablefrequency oscillatormeans having an output and operative for furnishing at said output atrain of triggering signals having a signal repetition frequency, androtor-speed-responsive frequency-varying means connected to saidfree-running adjustable-frequency oscillator means and operative forcontinuously varying the frequency of said free-runningadjustable-frequency oscillator means in dependence upon the speed ofsaid rotor; and a ring counter ciruit having an input connected to saidoutput for receipt of triggering signals, and comprising a plurality ofcircuit stages each capable of undergoing transistions between a firstand a second state, and each stage being so connected to a respectiveone of said stator winding means as to effect energization of therespective one of said stator winding means when in said second state,and wherein said stages are connected together in a circle in such amanner that upon receipt of successive triggering pulses at said inputsuccessive ones of said stages undergo a transition to said secondstate, so that successive ones of said stator winding means areenergized in sequence to create a rotating magnetic field.
 2. A motor asdefined in claim 1, wherein said free-running adjustable-frequencyoscillator means consists of a free-running adjustable-frequencyoscillator having an input for receipt of a D.C. signal and is operativefor furnishing a train of triggering signals having a signal repetitionfrequency dependent upon the amplitude of such D.C. signal, and whereinsaid frequency-varying means fuRther comprises speed signal means forapplying to said input of said free-running adjustable-frequencyoscillator a D.C. signal indicative of rotor speed.
 3. A motor asdefined in claim 2, wherein said speed signal means consists of meansfor applying to said input of said free-running adjustable-frequencyoscillator a substantially flat D.C. signal having a level indicative ofrotor speed.
 4. A motor as defined in claim 1, said frequency-varyingmeans comprising means for producing an increase in said signalrepetition frequency in response to to an increase of rotor speed.
 5. Amotor as defined in claim 1, said frequency-varying means comprisingmeans for establishing a signal repetition frequency having apredetermined start-up value when said rotor is being started-up.
 6. Amotor as defined in claim 1, said triggering means comprising additionalfrequency-varying means for varying the signal repetition frequency ofsaid train at the will of an operator.
 7. A motor as defined in claim 6,said additional frequency-varying means comprising an adjustableRC-network.
 8. A motor as defined in claim 1, said signal repetitionfrequency having a start-up value when said rotor is being started-up,and said triggering means comprising means for adjustably determiningsaid start-up value.
 9. A motor as defined in claim 1, and furtherincluding speed regulating means for regulating the speed of said rotor.10. A motor as defined in claim 1, and further including shut-off meansfor terminating energization of successive ones of said winding meanswhen the rotor exceeds a predetermined value.
 11. A motor as defined inclaim 10, said shut-off means including voltage-threshold meansconnected with at least one of said stator for producing a shut-offsignal when the induced voltage in said at least one stator windingreaches a threshold value.
 12. A motor as defined in claim 11, saidvoltage-threshold means comprising Zener diode means.
 13. A motor asdefined in claim 1, said triggering means comprising a multivibrator.14. A motor as defined in claim 1, said rotor comprising permanentmagnet means.
 15. A motor as defined in claim 1, said circuit stagescomprising a plurality of current-furnishing power transistors, eachconnected with a respective one of said stator winding means.